Ammonium nitrate propellant with low flame temperature exhaust gases



United States Patent 3,138,497 AMMONIUM NITRATE PRGPELLANT WITH LOW FLAME TEMPERATURE EXHAUST GASES Don E. Kennedy, Palos Verdes Estates, Califl, assignor to Standard Gil Company, Chicago, Ill., a corporation of Indiana No Drawing. Filed July 18, 1962, Ser. No. 210,829

5 Claims. (Cl. 149-19) This invention relates to ammonium nitrate explosives suitable for use as gas generating compositions having relatively low exhaust gas temperatures.

Ammonium nitrate explosive compositions comprise the ammonium nitrate oxidizer, a catalyst for accelerating the decomposition of the ammonium nitrate and an oxidizable material which utilizes the excess free oxygen produced in the decomposition of the ammonium nitrate. These oxidizable materials commonly have included virtually all classes of compounds and elements which can react with oxygen. Usually, these oxidizable materials are organic compounds or polymers or mixtures of organic compounds which function as a matrix former or binder by holding the normally, finely divided ammonium nitrate particles and catalyst into a predetermined stable configuration. The stable configuration may be of particular shapes such as cylinders, tubes, discs and cubes; also, they may be irregular shapes obtained by crushing larger blocks to obtain a spectrum of particle size. The configuration must have dimensional stability over the range of ordinary atmospheric temperatures for the usual storage times required by military service.

For explosive purposes, or rocket propulsion purposes, the temperature of the combustion gases is not critical and usually higher temperatures are desired since they provide higher volume and/or higher pressure product. Recently many uses have been made of ammonium nitrate type explosive compositions as sources of combustion gases at elevated pressure for the purpose of driving turbines which in turn provide auxiliary power for uses in rocketry, or other high demand, limited time requirements. For this gas generation utility, the exhaust gases from the combustion chamber must be passed through conduits to the point of ultimate use. The gas generation period may extend from a very few seconds to as much as several minutes. In the more extended time durations, it is evident that the conduits carrying the exhaust gases can become heated to high temperatures such that significant reductions in metal strengths takes place with a consequent requirement for either more metal present, or the use of expensive alloys. The desire for light weight gas generator installations, and at the same time a minimum of capital cost, has lead to a seach for ammonium nitrate explosive compositions providing exhaust B gas of much reduced temperatures over those presently detained. The principal object of this invention is an amimonium nitrate explosive composition suitable for gas generating purposes which does provide exhaust gases of ma kedly lower temperature than provided heretofore by knthwn compositions. Other objects will become apparent tin the course of the detailed description of the inventitpn.

The gas generating composition of this invention may be des czribed as a conventional composition consisting essentialliv of a finely divided ammonium nitrate as the predominant component, a finely divided catalyst for acice celerating the decomposition of ammonium nitrate and an organic binder material for maintaining the composition in a stable configuration wherein there have been incorporated discrete granules of ammonium oxalate, dispersed substantially uniformly throughout the mass of the composition. Sufficient ammonium oxalate is present to decrease significantly the temperature of the exhaust gases from the combustion of the improved composition relative to the temperature of exhaust gases produced by the conventional composition in the absence of the dispersed ammonium oxalate granules.

The amount of ammonium oxalate which is present in the composition of the invention is determined by the temperature requirements of the particular exhaust gases and the oxidizable materials present in the composition. It has been observed that substantial temperature reduction is obtained with 1% and in many cases, even less of ammonium oxalate present. In most instances, 15% is sufiicient to meet even severe requirements of temperature decrease. In gas generating compositions wherein the organic binder material consists of organic polymeric material and organic compound containing combined oxygen (as a plasticizer usually), it has been found that temperatures on the order of 1,000-l,500 F. are readily obtainable using about 5-15 weight percent of dispersed ammonium oxalate granules.

It is to be understood that herein all percentages are weight percent.

The ammonium oxalate is present in the gas generating compositions as discrete granules surrounded by the homogeneous body of ammonium nitrate catalyst and organic binder material. In practice, all of the ingredients of the gas generating composition are intermingled to form the homogeneous propellant body; then to this premixed body portion held in a semi-plastic condition, the solid granules of ammonium oxalate are added and the body portion and the granules are intermingled for a period of time to substantially uniformly disperse the discrete granules throughout the body of the gas generating composition. The mixing is held to a minimum in order to avoid breaking down the granules charged to the op eration.

The ammonium oxalate granules are macro sized in comparison to the finely divided ammonium nitrate and finely divided combustion catalyst. The ammonium oxalate may be crystalline or in irregular shapes from the crushing of larger masses or cakes of oxalate. Preferably, the granules are deliberately shaped from fines; these shapes commonly are rods, cylinders or tablets or spheroids. Spheroids having a diameter in the range of about 110 millimeters are particularly suitable. Very good results have been obtained with cylindrical shaped pills having a diameter of about 1-8 millimeters and a length of about 1-8 millimeters.

Any of the conventional catalysts known to accelerate the decomposition of ammonium nitrate may be present. Illustrative of these catalysts are ammonium chromate, copper chromite, ethylene diamine chromate, cobalt cyanide, copper cyanide, nickel cyanide, copper cyanamide, soluble Prussian blue, insoluble Prussian blue, alkali metal barbiturates, alkali metal anthranilates, alkali metal glutamates, pyrogene blue, methylene blue, etc.

The catalyst is present in the composition in a finely divided state. Most of the known catalytic materials are prepared in the form of fine powders. The catalyst is present in the composition in an amount to produce the desired acceleration of the decomposition of ammonium nitrate (burning rate promoters may also be present to get greater decomposition rates at a given catalyst contenta particularly good burning rate promoter is finely divided carbon such as carbon black). For most purposes, about 1-5% of combustion catalyst is suitable and will be present in the composition.

The ammonium nitrate may be the high purity material commonly produced by synthetic plants today, or it may be technical grade containing small amounts of inorganic impurities. In addition to the ammonium nitrate, for special purposes, sodium nitrate or potassium nitrate may be present in an appreciable amount. The decomposition rate of the ammonium nitrate is influenced by the particle size. For gas generation purposes, the ammonium nitrate is finely divided. Particularly suitable ammonium nitrate will contain about 80 weight percent of material having a screen size greater than 80 mesh and smaller than 30 mesh. The more finely powdered ammonium nitrate is used where higher burning rates are desired.

The ammonium nitrate is the predominant component of the composition and usually will be present in amounts in excess of 60%. The ammonium nitrate content will vary dependent upon the number of other ingredients present in the composition. When oxygen balance is of importance, the production of a soot free exhaust gas is attained by combination of ammonium nitrate present and the use of organic binder materials having lower oxygen demands, i.e., organic compounds containing combined oxygen.

The organic binder material performs not only the function of maintaining the composition in a stable configuration, but also provides additional energy by utilizing the free oxygen released by the decomposition of the ammonium nitrate. Substantially any organic material which has the capacity of forming a matrix enclosing the solid ingredients and providing strength and resistance to deformation by its own weight at ordinary atmospheric temperatures may be used as a binder material. It is conventional to use organic binder materials Which form thermoset solids after being intermingled with the solid ingredients. Illustrative of these are the synthetic rubbers such as Thiokol rubber, and the polybutadienes. Other elastomeric materials may be used which can be brought into a plastic state permitting thorough intermingling with the solid ingredients, for example, natural rubber, GRS rubber, and butyl rubber. The hydrocarbon polymers may be used, for example, polyethylene, polypropylene, and polystyrene. Natural hydrocarbons such as asphalt and high melting point waxes may be used. It is to be understood that these organic polymers whether elastomers or crystalline in nature may be used in conjunction with other organic compounds which function as plasticizers for the polymeric materials.

Many synthetic organic polymeric materials which are not suitable for use as binders alone may be utilized when converted to a thermoplastic material by means of a suitable plasticizer. It has been observed that the best plasticizers for organic polymeric materials are organic compounds containing combined oxygen and/ or combined nitrogen.

Particularly suitable synthetic polymeric materials are the cellulose esters and ethers such as cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, and ethyl cellulose. The polyvinyl resins are particularly good Where very strong binders are desired; illustrative of these are polyvinyl chloride, polyvinyl acetate, polyvinyl propionate and copolymers thereof. The polyacrylates and polyacrylonitriles are other suitable polymeric base materials.

, In general, any material which is capable of plasticizing the polymer may be used in forming the binder material.

The best plasticizer will be determined by the specifications imposed on the particular gas generating composition. It is common to use a mixture of plasticizers.

Illustrative of the sweep of organic compounds containing combined oxygen (as well as combined nitrogen) is the listing set out at column 2 of U.S. Patent No. 3,012,- 867.

It is to be understood that the amount of binder material present will be determined by the requirements for the present composition. However, it has been observed that very broadly when the binder material consists of organic polymeric material and organic compound containing combined oxygen adapted to plasticize the polymer, about 15-35% of the binder material will be present.

Outstanding results over a broad range of military demands for gas generating compositions have been obtained with a composition consisting of (A) a body portion consisting of about 811% of celllose acetate, about 10-13% acetyl triethyl citrate, about 9-12% of triethyl citrate, 13% of finely divided carbon, about 0.5-1.5 of aromatic amine, about 0.00.5% of phenyl morpholine and about 15% of alkali metal barbiturate and the remainder essentially finely divided ammonium nitrate, and (B) granules of ammonium oxalate dispersed substantially uniformly throughout said body, said oxalate being present in an amount of about 5-15 of said composition, and said granules being substantially cylindrical in shape having a diameter of about 18 millimeters and a length of about 1-8 millimeters.

It is to be understood that in addition to the ingredients which have been spelled out in detail above, the gas generating compositions of the invention may include one or more of the conventional additives found in ammonium nitrate explosives and propellants. Commonly, these compositions will include an aromatic amine stabilizer such as toluene diamine; these aromatic amine gas inhibitors are discussed in U.S. Patent No. 2,942,963. The composition may include phenyl morpholine and the like materials as stabilizers; these materials are disclosed in detail in U.S. Patent No. 3,020,180. Various oximes may be present in these compositions such as are disclosed in U.S. Patent No. 2,973,255. Surfactants may be present to improve properties of the composition; for example, see U.S. Patent No. 2,946,671. The use of carbon as a burning rate promoter is discussed at length in U.S. Patent No. 2,942,963.

Illustrations Two gas generating compositions were prepared using formulations previously known according to techniques previously developed. These formulations were prepared by introducing the organic binder materials into a heated kettle provided with a mechanical stirrer. The binder materials were raised to a temperature where they became a viscous homogeneous liquid. The catalyst and other additives were blended into the molten binder before the addition of the solid ammonium nitrate. During the blending of the ammonium nitrate, the temperature of the mixing operation was held below 130 C. In this instance, the ammonium nitrate was of a screen size: passing through a 14 number screen and retained by a 30 num her screen; about 12% retained by an 80 number screen and passing through a 30 number screen; about 5% smaller than 80 number screen and the remainder larg than 14 number screen.

For purposes of comparison, compositions were {Jrepared wherein finely powdered ammonium oxalate ,Was added to the semi-plastic mix of the conventional e .plosive above and the mixing operation continued untii the ammonium oxalate powder had disappeared to this eye into the total mixture. Compositions were prepared to illustrate the invention by adding to the semi-plastic mass of the conve'entional compositions above ammonium oxalate pills vtgl 'lich had been prepared by compacting ammonium oxalate powder.

Body Formulations Two formulations were used which differed only in the organic binder material. This seeming small difference between the two formulations introduces a substantial difference in exhaust gas temperature. The two formulations are set out in the table below and are identified as No. 4 and No. 7. The proportions are weight percent.

Ingredient No. 4 N0. 7

Ammonium nitrate 62. 24 62. 24 Cellulose acetate (Lacquer grade). 10. 40 10. 40 Acetyl tn'ethyl citrate 11.85 11.85 Dinitrophenoxyethanolv 10. 45 None Triethyl citrate None 10. 45 Carbon black 3.06 3.06 Disodium barbiturate 1. 00 1.00 Toluene diamine 1.00 1. 00

Gas generating elements were prepared by extruding the plastic mass to rods 2.5 inches in diameter by 6 inches long and weighing 4 pounds. In all the tests the gas generating element was provided with a restrictor coating on the cylindrical surface and one end so that the element, when ignited, burned cigarette fashion. These elements burned for 60-70 seconds depending upon the particular pressure and formulation of the element being fired.

Gas generator cartridges were prepared and were fired. A pressure-time trace was attained in each run and also the temperature within the gas generator cartridge was followed by means of thermocouples positioned inside.

The formulation identified as No. 4 gave an average pressure of 1020 p.s.i. and a measured exhaust gas temperature of l,940 F.-1,060 C. The formulation identified as No. 7 gave an average pressure of 600 p.s.i. and a measured exhaust gas temperature of 1,750 F.955 C.

Compositions of the Invention Compositions of the invention were prepared as described above by admixing with either formulation No. 4 or formulation No. 7 sufiicient of the described pills of ammonium oxalate to produce a composition containing 10% of the pills and 90% of the particular formulation as the body portion of the composition.

Composition A.-This composition had as the body portion formulation No. 4. In the firing of a gas generator cartridge using this composition as the generating element, the average pressure was 520 p.s.i. and the meas ured exhaust gas temperature was 1,550 F.-843 C. (In all of these firings, the same nozzle size was used so that the pressure measured within the gas generator cartridge was the equilibrium pressure of the particular formulation.) Duplicate runs were made in all cases. These runs show that the composition A lowered the exit gas temperature almost 400 F. lower than that of the body portion of the element.

Composition B.-This composition had as its body portion, the formulation described as No. 7 above. The pressure developed in the gas generator cartridge using composition B was 300 p.s.i. and the measured temperature of the exhaust gas was 1,000 F.538 C.

Crawford bomb data on test strands produced from the formulations showed that the burning rate of the body portion of compositions A and B was the same as that of formulations 4 and 7 respectively. The only effect of the pressure of the ammonium oxalate pills was in the reduction of the equilibrium pressure and temperature. It is self-evident that this pressure can be adjusted by using the proper nozzle size.

Tests were also carried out on an element using formulation No. 7 as the body portion and ammonium oxalate pills having a size one-eighth A2) inch by one-eighth 4;) inch. The temperature time trace of this element was somewhat more uniform than when using the larger pills. It is thought that the smaller pills permitted a more uniform dispersion. In other respects, there was no appreciable difference between the elements using the different size pills.

In tests on storage stability, hot aging, etc., it was found that the ammonium oxalate pills did not change significantly the characteristics of the body portion of the compositions as obtained on elements produced from formulations 4 and 7 respectively. In other words, all of the good features of the conventional formulation were retained in the gas generating element plus the very much desired lowering of the exhaust gas temperature, by the presence of the ammonium oxalate pills.

Comparative Formulations Gas generating elements were prepared for comparison purposes by admixing finely powdered ammonium oxalate into formulations 4 and 7 respectively. The ammonium oxalate was introduced in one set of test elements to the amount of 1%, and in another set of test elements, in the amount of 4%. In the case of the 4% containing elements, the elements could not be ignited under the same conditions which would ignite formulations 4 and 7 and compositions A and B.

The 1% containing comparison elements could be ignited and did give some reduction in exhaust gas temperature. However, it was observed that the stability of the composition was markedly decreased and the ability to maintain stable configuration in storage was suificiently impaired so that these formulations are not of value to gas generator use, at least for military rocketry purposes.

The above clearly establishes that the use of discrete granules of ammonium oxalate dispersed in ammonium nitrate gas generating compositions is a satisfactory procedure for simultaneously obtaining cooler exhaust gas temperatures while retaining the physical and chemical characteristics of the gas generating material whose exhaust gas temperature is desired to be lowered.

Thus having described the invention, what is claimed is:

l. A gas generating composition consisting essentially of finely divided ammonium nitrate as the predominant component, a finely divided combustion catalyst for said nitrate, an organic binder material for maintaining said composition in a stable shape, and discrete granules of ammonium oxalate dispersed substantially uniformly throughout said composition in an amount of about 5-15 weight percent of said composition, said ammonium oxalate granules being substantially cylindrical in shape and having a diameter of about 1-8 millimeters and a length of about 1-8 millimeters.

2. A gas generating composition having (1) a body portion consisting of about 1-5% of ammonium nitrate combustion catalyst, about 1535% of binder material consisting of organic polymeric material and organic compound containing combined oxygen, said compound being adapted to plasticize said polymer and the remainder, essentially, finely divided ammonium nitrate, and (2) discrete granules of ammonium oxalate dispersed substantially uniformly throughout said body, said oxalate being present in an amount of about 5-15 of said composition, said ammonium oxalate granules being substantially cylindrical in shape and having a diameter of about 18 millimeters and a length of about 1-8 millimeters.

3. The composition of claim 2 wherein said polymer is a cellulose ester.

4.'The composition of claim 2 wherein said catalyst is alkali metal barbiturate.

nitrate, and (B) granules of ammonium oxalate dispersed 10 w 8 substantially uniformly throughout said body, said oxalate being present in an amount of about 5-15 of said com position, and said granules being substantially cylindrical in shape having a diameter of about 1-8 millimeters and 5 a length of about 1-8 millimeters.

References Cited in the file of this patent UNITED STATES PATENTS 3,002,830 Barr V V V.,. Oct. 3, 1961 

1. A GAS GENERATING COMPOSITION CONSISTING ESSENTIALLY OF FINELY DIVIDED AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT, A FINELY DIVIDED COMBAUSTION CATALYST FOR SAID NITRATE, AN ORGANIC BINDER MATERIAL FOR MAINTAINING SAID COMPOSITION IN A STABLE SHAPE, AND DISCRETE GRANULES OF AMMONIUM OXALATE DISPERSED SUBSTANTIALLY UNIFORMLY THROUGHOUT SAID COMPOSITION IN AN AMOUNT OF ABOUT 5-15 WEIGHT PERCENT OF SAID COMPOSITION, SAID AMMONIUM OXALATE GRANULES BEING SUBSTANTIALLY CYLINDRICAL IN SHAPE AND HAVING A DIAMETER OF ABOUT 1-8 MILLIMETERS AND A LENGTH OF ABOUT 1-8 MILLIMETERS. 